(1) Field of the Invention
The present invention relates to a nanotube, a near-field light detecting apparatus and a near-field light detecting method which are capable of detecting near-field light (evanescent light). The present invention also relates to an apparatus for analyzing or observing a surface state (shape, characteristics or the like) of a sample by use of near-field light.
(2) Description of the Prior Art
When light is incident on a boundary surface which has a varying refractive index at an angle which causes total reflection (generally, at an angle of refraction equal to or larger than 90 degrees), the incident light is totally reflected on the boundary surface (reflection plane), in which case the light exudes to the opposite side of the reflection plane. This exuding light is called “near-field light.” Other than the foregoing, the near-field light also includes light which exudes from a miniature aperture smaller than the wavelength of the light, through which the light is passed.
The near-field light can be utilized to analyze a surface state (shape, characteristics or the like) of a sample such as semiconductor materials, organic or inorganic materials, vital samples (cells) and the like. An ordinary optical microscope cannot measure a sample at a resolution higher than the wavelength of light due to diffraction of the light. This is called “diffraction limit of light.” Since an analysis utilizing near-field light permits measurements at a resolution exceeding the diffraction limit of light, a variety of analyzers based on near-field light have been under investigation in recent years.
A near-field optical microscope is known as one of the state analyzers which utilize near-field light. In dominating near-field optical microscopes, glass fiber is used to constitute a part (opening) which receives near-field light produced on the surface of a sample. In such a near-field optical microscope, near-field light emitted from the surface of an object under measurement is transformed into ordinary light which is led to an optical signal processing unit through the glass fiber. Then, the ordinary light is converted into an electric signal in the optical signal processing unit for signal processing. In this event, since the near-field optical microscope must have an opening sized to be equivalent to the wavelength of the light for the transformation of near-field light into ordinary light and the propagation of ordinary light through the glass fiber, the opening must be sized on the order of microns.
Other than the foregoing near-field optical microscope, JP-A-10-170523 describes a scanning probe microscope. FIG. 1 generally illustrates the configuration of the scanning probe microscope.
Quartz plate 92 which carries sample 83 on the surface thereof is fixed on holder 93. Objective lens 94, light chopper 96 and light source 95 are disposed on the back side of quartz plate 92. Light from light source 95 sequentially passes through light chopper 96 and objective lens 94, and is irradiated onto the back of quartz plate 92 at a predetermined incident angle, i.e., at an angle at which near-field light is produced.
Probe cantilever 81 is disposed opposite to the surface of sample 83. Probe cantilever 81 is supported by three-dimensional driving mechanism 85 through laminated piezo element 82, and has a portion connected to capacitor sensor 90 through copper line 89. The output of capacitor sensor 90 is supplied to lock-in amplifier 91.
Three-dimensional driving mechanism 85 comprises semiconductor laser 86 for illuminating a predetermined part of probe cantilever 81, and bisected photodetector 87 disposed in a direction in which light from semiconductor laser 86 travels after it is reflected at the predetermined part. The output of bisected photodetector 87 is supplied to lock-in amplifier 88.
Laminated piezo element 82, which vibrates probe cantilever 81 in a direction perpendicular to the surface of sample 83, is supplied with a voltage signal from signal generator 84 for controlling the vibrations. Signal generator 84 supplies a reference signal at a preset frequency to lock-in amplifiers 88, 91, and supplies an operation command signal at a preset frequency to a light chopper 96, in addition to supplying the voltage signal to laminated piezo element 82. Each of lock-in amplifiers 88, 91, three-dimensional driving mechanism 85, and signal generator 84 is connected to controller 97.
FIG. 2 generally illustrates probe cantilever 81. Probe cantilever 81 comprises support 121 made of Pyrex glass; lever 126 having one end supported by support 121; and probe 127 formed at the other end of lever 126. Probe 127 is made up of silicon nitride 122, metal film 123 and photoconductive film 124 laminated in sequence with a V-shaped cross section. Photoconductive film 124, even though it is an insulating material, when it is not irradiated with light, behaves as a conductor in area 125 at the leading end of probe 127 (pointed end) when near-field light from sample 83 is supplied to area 125. Because of this characteristic of photoconductive film 124, near-field light incident on area 125 causes a change in the capacitance between metal film 123 and a surrounding conductor in probe cantilever 81. This change in the capacitance follows the intensity of the incident near-field light.
The scanning probe microscope illustrated in FIG. 1 takes advantage of a change in the capacitance due to the difference in the intensity of near-field light in probe cantilever 81 to observe the surface of sample 83 in the following manner.
Piezo element 82 vibrates probe cantilever 81 at a frequency ω1, near its resonant frequency, causing lock-in amplifier 88 to measure the amplitude of vibrations of a component at frequency ω1 in lever 126 from the result of a detection made by bisected photodetector 87. Controller 97 scans probe cantilever 81 on the surface of sample 83 as it controls three-dimensional moving mechanism 85 to move probe cantilever 81 such that the component at frequency ω1 presents a constant amplitude of vibrations. This control results in a constant distance held between probe 127 of probe cantilever 81 and the surface of sample 83.
Light from light source 95, on the other hand, is modulated by light chopper 96 at a frequency ω2. The modulated light is irradiated to the back of quartz plate 92 to produce near-field light on the surface of sample 83. Then, probe cantilever 81 is scanned on the surface of sample 83 to detect a change in the intensity of the near-field light in an in-plane direction (intensity distribution). Specifically, the change in the intensity of the near-field light is detected by lock-in amplifier 91 as a component at frequency (ω2–ω1) within a change in the capacitance in probe cantilever 81.
Controller 97 creates an image indicative of the intensity distribution of the near-field light on the surface of sample 83 from the result of the detection made by lock-in amplifier 91, and displays the image on a display device, not shown.
Other than the foregoing microscope, JP-A-2001-281124 describes a probe having a carbon nanotube for use with a scanning near-field optical microscope (SNOM). FIG. 3a generally illustrates the configuration of the SNOM probe, and FIG. 3b illustrates a cross-sectional view taken along a line A–A′ in FIG. 3a. 
As illustrated in FIG. 3a, the SNOM probe has a cantilever 102 which extends from cantilever base 101 in a predetermined direction, and a tip (probe) 103 at the leading end of cantilever 102. As illustrated in FIG. 3b, tip 103 is a hollow element in the shape of a quadrangular pyramid which has a square face with the dimensions of 1 μm×1 μm at the leading end. An aperture 103a is formed through the square face around the center thereof. A mounting groove 104a is formed on an edge of the leading end of tip 103 for mounting carbon nanotube 104 which extends substantially in the vertical direction from the square face at the leading end. Carbon nanotube 104 is electrically conductive.
A scanning probe microscope using the foregoing SNOM probe involves two operations: a first operation as an atomic force microscope (AFM) which uses a carbon nanotube, and a second operation as a scanning near-field optical microscope which uses a tip 103. In the first operation, cantilever 102 of the SNOM probe is vibrated near the resonant frequency, and the distance between carbon nanotube 104 and sample 83 is controlled by an actuator such as a piezo element such that the amplitude of vibrations receives constant attenuation or a phase shift. This is used to measure asperities on the surface of sample 83. In the second operation, the vibration of cantilever 102 is stopped, and sample 83 is scanned while the distance between opening 103a of tip 103 and the surface of sample 83 is controlled with reference to the result of the measurement (asperity information) provided by the first operation. Simultaneously, light is converged to aperture 103a from the back of tip 103 to produce near-field light in the vicinity of aperture 103a, and the surface of sample 83 is excited by the produced near-field light for observation.
When glass fiber is used to constitute the part (opening) which receives near-field light produced on the surface of a sample, the spatial resolution depends on the size of the opening. Since the glass fiber must propagate ordinary light transformed from near-field light, it is impossible to reduce the diameter of the glass fiber to the wavelength of ordinary light or less. Since the size of the opening corresponds to the diameter of the glass fiber, the conventional microscope cannot provide spatial resolution higher than the wavelength of ordinary light. For this reason, when near-field light is utilized to observe the surface of a sample, for example, a VLSI (very large scale integrated circuit) device, an element which is machined using the VLSI manufacturing technology, and the like, the microscope fails to provide a spatial resolution high enough to identify a part at which near-field light is produced. In addition, when near-field light is utilized to evaluate materials, it is difficult to locate a near- field light emitter at a molecular level.
In the scanning probe microscope described in JP-A-10-170523, the spatial resolution is determined by the size of area 125 at the tip of probe 127 (pointed end) of probe cantilever 81, so that the spatial resolution can be increased by reducing area 125. However the silicon nitride 122, which forms part of area 125, must be generally doped with a certain amount of impurities, so that in this structure, a sufficient thickness should be ensured for silicon nitride 122 in order that near-field light incident on the silicon nitride 122 to permit the silicon nitride to act as a conductor. Since there is a restriction on the reduction in the size of area 125, it is likewise difficult to accomplish a spatial resolution high enough to permit the identification of a part at which the near-field light is produced and the location of a near-field light emitter at a molecular level. Also the silicon nitride 122, when extremely thinned down, would give rise to another problem in that area 125 could be partially damaged when probe 127 comes into contact with the surface of a sample.
With the SNOM probe described in JP-A-2001-281124, the spatial resolution is determined by the size of aperture 103a in tip 103. Although the spatial resolution can be increased by reducing aperture 103a, it is again difficult to achieve a spatial resolution high enough to permit the identification of a part at which near-field light is produced, and the location of a near-field light emitter at a molecular level.